1. Field of the Invention
The present invention relates to a bandgap type reference voltage source with low supply voltage.
2. Description of the Related Art
In most electronic devices with a high integration scale, there are analogue blocks which require a reference voltage that is independent of the temperature and of the supply voltage. Examples of these electronic devices are voltage regulators for programming and erasing non volatile memories and DC/DC voltage reduction converters which generate internal supply voltages regulated at a fixed value.
The generation of reference voltages is generally obtained through a source circuit which supplies a bandgap output voltage.
Various bandgap reference sources are known. The simplest is formed by bipolar transistors, present in standard CMOS technology, of a vertical type, as shown in FIG. 1.
The bandgap source 1 of FIG. 1 comprises a bandgap stage 18, an operational amplifier 15 of transconductance type, and an output stage 19.
The bandgap stage 18 comprises a first and second branch 2, 3 flowed by a first and a second current I1, I2. The first branch 2 is formed by a first PMOS transistor 5 and by a first diode connected bipolar transistor, shown in FIG. 1 as a diode 6; the second branch 3 is formed by a second PMOS transistor 7, by a first resistor 8 and by a second diode 9. The PMOS transistors 5, 7 are identical, have source terminals connected to a supply line 12, and drain terminals connected to a first and, respectively, to a second output node 10, 11. The output nodes 10, 11 are set respectively at voltages VA, and VB. The first output node 10 is connected to an anode terminal of the first diode 6; the second output node 11 is connected to an anode terminal of the second diode 9 through the first resistor 8. The diodes 6, 9 have an area ratio 1:n and have their cathodes connected to ground 16. The first resistor 8 has a resistance R1.
The operational amplifier 15 has an inverting input connected to the first output node 10, a non-inverting input connected to the second output node 11 of the bandgap stage 18 and an output connected to the gate terminals of the PMOS transistors 5, 7.
The output stage 19 comprises a PMOS output transistor 20, an output resistor 21 and an output diode 22. The PMOS output transistor 20 is equal to the first and second PMOS transistors 5, 7 (and thus it is formed using the same technology and has the same dimensions as the transistors 5, 7) and has source terminal connected to the supply line 12, gate terminal connected to the output of the operational amplifier 15, and drain terminal defining an output terminal 24 on which there is a bandgap voltage VBG. The output terminal 24 is connected, through the output resistor 21, to the anode of the output diode 22, the cathode of which is connected to ground 16. The output resistor 21 has a resistance R2; on the output diode 22 there is a voltage VD and in the output stage 19 flows a current I3.
Since the PMOS transistors 5, 7 are identical and have the same gate-to-source voltage Vgs, this gives:
I1=I2,
moreover the operational amplifier 15 maintains VA=VB.
When the equations of the dipole 13 formed by the first diode 6 and of the dipole 14 formed by the resistor 8 and by the second diode 9 are written, the conditions of equality of current and voltage indicated above occur only when:
I1=I2=(VT/R1)ln(n).xe2x80x83xe2x80x83(1)
Moreover, as the PMOS output transistor 20 is identical and has the same gate-to-source voltage Vgs as the first and the second PMOS transistor 5, 7, it conducts a current I3=I1=I2.
Consequently, in the PMOS transistors 5, 7, 20 there flows a current proportional to VT/R. The bandgap voltage VBG present on the output terminal 24 is therefore equal to:
VBG=VD+I3* R2=VD+K(VT/R1)R2xe2x80x83xe2x80x83(2)
In (2), the resistance ratio R2/R1 is insensitive to temperature variations, since the two resistors 8, 21 vary in the same way; vice versa the terms VT and VD are variable with temperature. However, by acting on the coefficient K (through the mirroring ratio n) and on the number of diodes in parallel, it is possible to ensure that the temperature variations of VT and VD are compensated and that the bandgap voltage VBG present on the output terminal 24 is substantially insensitive to temperature.
The circuit in FIG. 1, however, has the problem that the inputs of the operational amplifier 15 have a temperature dynamics of 300 mV (xe2x88x922 mV/xc2x0 C.) and consequently, when the power supply falls below 1.5 V, the operational amplifier 15 does not work correctly. In fact, on the outputs of the operational amplifier 15 there are transistors (whether of the N-type or the P-type) which, at least in certain temperature intervals, work below threshold.
Moreover the bandgap voltage VBG generated by the output stage 19 is equal to about 1.25 V, so the supply voltage must be kept above 1.5 V.
Another known bandgap type reference source uses NMOS transistors operating below threshold instead of the first and the second diode 6, 9. This solution solves the problem of operation at a low supply voltage as regards the bandgap stage, but it suffers from other problems. In fact its PSSR value (Power Supply Rejection ratio) in DC is not very high; consequently, a supply voltage decrease leads to an unacceptable variation of the output voltage. Moreover, in a dynamic condition, the rejection of the noise coming from the power supply is not very good. Finally, also this solution uses an output stage similar to that of FIG. 1, so it is affected by the same problem of limitation of the minimum usable power supply voltage.
An embodiment of the invention solves the problems affecting the known bandgap reference sources.
According to an embodiment of the present invention a bandgap type reference voltage source and a method for generating a reference voltage in a bandgap type reference voltage source are provided.
A bandgap type reference voltage source using an operational transimpedance amplifier is provided. The bandgap stage is formed by a first and a second bandgap branch, parallel-connected. The first bandgap branch comprises a first diode and a transistor, series-connected and forming a first output node; the second bandgap branch comprises a second diode and a second transistor series-connected and forming a second output node. The operational transimpedance amplifier has inputs connected to the output nodes of the bandgap stage. An amplifier current detecting stage is connected to the outputs of the operational amplifier and supplies a current related to the current drawn by the operational amplifier. A diode current detecting stage is connected to the output of the amplifier current detecting stage and to an output of the operational amplifier and supplies a current related to the current flowing in the first diode. An output stage transforms this current into a stabilized voltage.
A method of operation is also provided.